<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>

        // variable: 可变的
        var x = 1; // 数字: number
        var y = 2;
        var z = x + y;

        var msg = 'hello world'; // 字符串: string

        alert(msg);

    </script>
</body>
</html>